This invention relates to an engine starter motor and more particularly to a vehicular starter motor.
FIG. 5 is a sectional view of a clutch arrangement of a conventional engine starter motor. In FIG. 5, 2 is a plunger, 3 is a shift lever rotatable about a fulcrum 3a and engaged at its upper end with the plunger 2 and at its lower end with the rear end of an over-running clutch unit 4. Reference numeral 5 is an electric motor, 6 is an armature and 7 is a field coil for generating a magnetic field in the armature 6. Reference numeral 8 is an output rotary shaft extending from the rotary shaft 6a of the armature 6, and a clutch outer member 4a of the over-running clutch unit 4 is in engagement with helical splines 8a formed in the outer circumference of the rotary shaft 8. Reference numeral 9 is a pinion capable of engaging a ring gear of an unillustrated engine and is integral with the clutch inner member 4b. 10 is a bearing for supporting the pinion 9 on the output shaft 8. 11 is a stopper mounted at the front end portion of the output rotary shaft 8 by a ring 11a, and the front end of the pinion 9 abuts against this stopper 11. 12 is a front housing, and 13 is a bearing mounted to the front housing 12 for supporting the front end of the output rotary shaft 8. 14 is a washer for receiving a thrust force of the output rotary shaft 8. Reference numeral 2a is a spring for providing a pushing force to the pinion 9 when it is to be engaged with the ring gear.
The operation will now be described. When a key switch (not shown) is turned on, the excitation coil of the solenoid switch 1 is energized to attract the plunger 2 rearwardly (to the left). This causes the upper end of the lever 3 engaged with the plunger 2 to be pulled leftward in the figure and the lower end of the lever 3 to be moved rightward about the fulcrum 3a, thus causing the pinion 9 together with the over-running clutch unit 4 to slide along the output rotatable shaft 8 to bring it into engagement with the engine ring gear (not shown). On the other hand, at the same time as the above operation, since the solenoid switch is closed, the electric motor 5 is energized to rotate the armature 6. This rotation of the armature is transmitted from the rotary shaft 6a to the pinion 9 through the helical splines 8a and the over-running clutch unit 4, thereby rotating the engine ring gear to start the engine.
While the conventional engine starter motor is constructed as above described, a reduction of the size of the starter motor is desired in the circumstances that the number of auxiliary equipments around the vehicular engine is increased and the fuel comsumption rate should be decreased. One measure for reducing the size of the starter motor is an increased gear ratio between the pinion 9 and the ring gear, because the starter motor can be made light-weight by selecting a larger gear ratio. The gear ratio can be increased either by increasing the number of teeth the engine ring gear or by decreasing the number of teeth of the pinion.
However, increasing the number of teeth of the engine ring gear is difficult because this means an increased diameter of the engine ring gear which in turn causes the diameter of the engine transmission to be large, thus making the engine compartment of the vehicle large.
Also, decreasing the number of teeth of the pinion is not practical for the following reasons. In the conventional engine starter motor, the output rotary shaft 8 is located inside of the pinion 9, and in view of the mechanical strength of the output rotary shaft 8, the strength of the dedendum of the teeth of the pinion, and of the reduction of the impact and wear of the pinion by making the engagement ratio between the pinion and the ring gear as large as possible, the number of teeth of the pinion cannot be decreased as much as desired. More particularly, the Module (pitch divided by pi) of the pinion is equal to or less than 2.75 and generally a pinion for an ordinary automobile has a Module of 2.54. The minimum number of teeth of such a pinion is eight, and if the number of teeth is decreased to less than eight, the contact ratio between the pinion and the ring gear is decreased, resulting in a large impact applied to various parts of the stater motor and the engine ring gear and a large pinion wear. Providing sufficient mechanical strength is thus difficult, making such thus the pinion impractical.